Indoor air quality is becoming an increasingly important issue in today's building environment, particular in commercial office buildings. In order to provide guidance to building and HVAC system designers, the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) has specified minimums for ventilation rates and indoor air quality that will be acceptable to human occupants and will avoid adverse health effects. These requirements were published in ASHRAE Standard 62-1989. This standard, entitled "Ventilation Standard for Acceptable Indoor Air Quality", presents a series of general requirements for ventilation systems and two alternative procedures for providing acceptable indoor air quality.
Outside ambient air is brought into the zone to correct for unsatisfactory indoor air quality. Outside air may be either substantially colder than the desired indoor air temperature or may be substantially hotter than the desired indoor air temperature. In either case the heating, ventilation, and air conditioning (HVAC) system that serves the particular zone is going to have to work harder to set the desired temperature in the zone when outside ambient air is brought into the zone in substantial quantities. This results in energy inefficiencies for the building. It is desirable in all cases to minimize the amount of outside air that is brought into the zone consistent with maintaining a satisfactory air quality in the zone.
The first such procedure for controlling indoor air quality in the ASHRAE standard is the Ventilation Rate Procedure and uses ventilation air of specified quality in a fixed quantity for the specific zone to achieve acceptable indoor air quality in the zone. The Indoor Air Quality Procedure, on the other hand, uses sensing of known and specifiable contaminants to control the amount of ventilation in order to achieve acceptable indoor air quality in the zone. In typical circumstances, the ventilation system is designed according to the "Ventilation Rate" or "Prescriptive" method, i.e., provide 30 cubic feet per minute (CFM) per person for N persons where N is the design occupancy level of the zone.
The Ventilation Rate Procedure does not use direct sensing of air quality. Instead, it defines the amount of outside air to be delivered to the space based on the design occupancy level of the zone. This method by itself does not insure acceptable air quality and frequently results in inappropriate higher energy costs, since the procedure requires an assumption of the worst indoor air quality and ventilates to correct for this condition at all times, even when the air quality in the zone is satisfactory.
The "IAQ" or "Performance" procedure allows modulation of the ventilation rate when occupancy falls below the design level. This procedure is to directly measure indoor air quality and control the amount of outside air accordingly. Utilization of the IAQ procedure reduces energy costs for the building by increasing outside air flow only when there is a specific need. The drawback to this approach has been the lack of availability of good, low cost air quality sensors and the fact that designers cannot comply until the building is actually occupied.
It is important to note that, utilizing current equipment, system designers can approach compliance with the Ventilation Rate Procedure but cannot comply with the more energy efficient IAQ Procedure that are specified in Standard 62-1989.
Up to now, two different sensing techniques have been used to determine air quality when attempting to meet the IAQ Procedure standard. The first is CO.sub.2 sensing and the second is VOC sensing. Each sensor does an adequate job on its own of sensing the parameter that it is designed to sense, but is inadequate in determining true air quality in the zone. In addition, these sensors have been expensive, bulky, and lack long term stability, requiring frequent calibration.
CO.sub.2 concentration alone has, in the past, been utilized to regulate the amount of ventilation that is brought into a particular zone. CO.sub.2 is generally seen as an indicator of the occupancy and activity by humans of the zone , since it is the occupants and their activity that generate the CO.sub.2. It is desirable to increase ventilation when the concentration of CO.sub.2 reaches a certain defined level. Absent other contaminants, the modulation of the ventilation based solely on the concentration of CO.sub.2 is a reasonable procedure to ensure an adequate indoor air quality.
CO.sub.2 is not, however, the sole indicator of indoor air quality. As previously indicated, such items as the level of VOC's, temperature, and the relative humidity also are indicators of the indoor air quality. Of particular interest at the present is the level of VOC's. Such organic compounds typically escape from the fabric utilized in the furnishings of offices over a period of time. For example, the organic compound formaldehyde frequently contaminates the indoor air as it escapes from newly laid carpet. Organic compounds are also generated during the remodeling of spaces by glues, paints, and stains.
There are sensors that will very accurately sense a great number of individual organic compounds and accurately provide the concentration thereof. At present, such sensors are limited to very expensive laboratory instruments and are not practical for use in individual zones of buildings. Accordingly, the VOC sensing that is practical and cost effective for use in air conditioned zones is in general not very selective with respect to which organic compound is present and additionally does not provide a very accurate indication of the level of concentration. As a result of these limitations, reliance on VOC sensing within the zone as the sole indicator of the indoor air quality is not practicable.
It would be a distinct advantage in the industry to develop a low cost CO.sub.2 /IAQ sensor that is cost effective enough to be applied on a zone by zone basis. The IAQ sensor must sense contaminants other than CO.sub.2 and provide an output that reflects the sensing of those other contaminants. In addition, it must be available in both duct and wall mount versions. For the wall mount version, the size of the sensor must approximate the size of the common thermostat.
Additionally, such a low cost IAQ sensor could be modified to sense secondary indicators other than volatile organic compounds. In greenhouse applications, it is preferable to sense combinations using CO.sub.2 as the leading value combined with CO, Hydrocarbons or Ethylene. In fruit storage applications, CO.sub.2 may be combined with Ethylene or hydrocarbons. In tunnel applications, CO.sub.2 may be combined with CO or hydrocarbons. In pig farm applications, CO.sub.2 may be combined with Ammonia, CO or hydrocarbons.